What's Up In The Sky December 2010

The only time most of us think of the Earth having a shadow is during a lunar eclipse, when it covers the face of the Moon. Yet, every evening when the Sun goes down and the stars come out we are in it. And, what a shadow it is! It is as wide as the Earth from pole to pole 8,000 miles in diameter and going out into space as a long cone some 868,000 miles long, or about four times the Earth Moon distance. The length of the shadow isn't constant, it varies with our distance from the Sun. So it is longer in July when we are further from the Sun than in January when we are closer. That is why there are a few more total solar eclipses on average in July than at other times of the year. Earth's companion, the Moon, creates a shadow that falls about 20,000 miles short of the Earth when it is at apogee and extends some 19,000 miles past the Earth when the Moon is at perigee, so most solar eclipses are annular or partial. Since the cone of the Earth's shadow is still about 1/3 bigger than the Moon a total lunar eclipse can occur much easier.

On December 21 of this year we will be treated to a total lunar eclipse, see WHAT'S Up IN THE SKY for November. Those of us that live in the midwest will witness a rare total solar eclipse on August 21, 2017. This eclipse will come ashore on the Oregon coast and drop down across Nebraska. St. Louis will be just north of the the 71 mile wide path. The centerline of which will pass through Bloomsdale, Missouri, which is just north of Sainte Genevieve. Totality will be 2 1/2 minutes on the centerline. The event will continue on across southern Illinois, Kentucky, Tennessee and go out over the Atlantic at Charleston, South Carolina.

Although there can be up to five solar eclipses a year the average is two and no more than two can be total. For anyone spot on Earth the time span between total solar eclipses is 370 years. Which makes the next one over the same spot on April 8, 2024, rare and exciting.

If you would like to compute the Earth's shadow, or any other solar system member here is the formula.

L= r X Ro

Rs --Ro

Where r is the distance from the Sun, or star, to the occulting object. Ro is the radius of the object. Rs is the radius of the Sun, or star.The answer, L, is the length of the shadow.

Dec 1-2 At dawn the waning crescent of the Moon will be west of Saturn. On the 2nd west of Venus.

5 New Moon.

6 Mars will be occulted by waxing crescent Moon at dusk..

13 First Quarter Moon. Mars and Mercury about a degree apart low in the southwest sky at sunset.

Geminide meteor shower. The Moon will be low by midnight, peak for the shower is around 2am the 14th.